1. Field of the Invention
This invention relates to zoom lenses, and more particularly to zoom lenses of high optical performance suited for use in photographic cameras or video cameras.
2. Description of the Related Art
For objective lenses of the photographic cameras and video cameras, a better compromise of increasing speed and of minimizing the bulk and size has been sought. Particularly in the video camera, because the image pickup element has a relatively low sensitivity, it has been required that the speed be faster than 1:1.4 in F-number. Besides this, to allow for unification of the video camera with the video recorder in compact form, the requirement for reducing the size and weight of the objective lens should be fulfilled. It has further been required that the objective lens be of zoom type with high optical performance, and the image pickup element such as tube or CCD be preceded by a space of axial length long enough to arrange the protection glass plate and the various filters such as stripe filter for color separation and low pass filter therein, in other words, the back focal distance be sufficiently long.
To design an objective lens for adaptation to the video camera with the provision of such various features, it becomes difficult to preserve good optical performance. Particular difficulty is found in well correcting spherical aberration, coma and distortion. As a result, good contrast of image has to be sacrificed.
Also, what has generally been used in zoom lenses is the type shown in FIG. 1. The zoom lens of FIG. 1 comprises, from front to rear, a first lens unit 21 of positive power for focusing a second lens unit 22 of negative power monotonously movable for varying the image magnification, a third lens unit 23 of positive power whose reciprocating motion when expanded takes a convex locus toward the rear to compensate for the image shift which accompanies with the variation of the image magnification, and a fourth lens unit 24 of positive power having a function of forming an image. And, a diaphragm 25 is arranged in between the third and fourth lens units 23 and 24. However, because the space the diaphragm 25 occupies and the space in which the magnification varying lens unit moves do not overlap each other, the total length of the entire lens system tended to increase. Also, because the distance between the diaphragm and the first lens unit is very long, the diameter of the first lens unit, which is determined by the off-axial oblique pencil at or near the wide angle end of zooming range, tended to increase.
As the document of the related art, mention may be made of U.S. patent application Ser. Nos. 877,101 filed June 23, 1986 now U.S. Pat. No. 4,763,998 and 931,537 filed Nov. 14, 1986 assigned to the assignee of the present invention, in which the diaphragm is arranged in a space between the lens units movable for zooming. The zoom lens of Ser. No. 391,537 has a tendency of giving larger F-numbers when in the telephoto positions than when in the wide angle positions.